required amount of light is not coming inside the room
You can use whatever strength value for the environment that works for you, so if 20 is too dark, you can use 200, if 200 is too dark, you can use 2000, so if there is not enough of light, just add more light.
One thing to keep in mind is that if you have glass windows, whatever light comes through them, becomes refractive caustics, and they are terribly slow to calculate and not accurate at all in Cycles. You should consider using fake glass shader with transparent shader mixed with glossy for reflections by amount determined by Fresnel node. It might be a good idea to avoid calculating shadows from the glass at all, because they practically do not contribute to the realism of the render at all in vast majority of cases, so you can have the glass of all the windows as a separate object and disable it's visibility for all rays except camera rays:

What's important in lighting is not the numerical values, you use, but their proportions compared to each other. The light of the sun on a clear day might illuminate the ground with 120 000 lux while artificial lighting on a desk in an office is required to be(and often is) around 200 lux(at least where I am). So that's the difference between a light bulb and the sun in reality... Keep that in mind. In a dark painted room like yours the difference between the inside and the outside would be huge as well. No need to be afraid of that. If you have the values their relationships in the scene set up the way it is not convenient for you to adjust the strength of the environment and keep the ratios, you can also adjust exposure here:

... or here:

Both are OK, the first multiplies the values in the rendered result, the second multiplies the values of all the lights in the scene if I am not mistaken.
You should use portals to speed up the render - the noise clears up faster if you use portals, but they should not have any influence on the looks of the render.
Note that you definitely do not need any add-ons whatsoever to have nice light in your scene. They might make it more convenient for you to set things up, but are completely optional.
Here is an example of how I usually set it all up(.blend file (134.03MB)). I hope that might be useful to have a look. Feel free to use all assets in the file under CC0 license if you wish.